Self-Cooling Walls

The walls use hydrogel bubbles inside a ceramic framework. Image: IAAC.

The world of The Tangle is full of seemingly futuristic technology specifically designed for environmental sustainability. One technology mentioned in passing is self-cooling walls. The idea is that buildings of the future have in-built sustainable mechanisms for cooling and heating instead of requiring expensive and energy intensive air-conditioning.

Like much of the technology in the book, the idea and even prototypes for this technology already exists.

A research institute in Spain, the Institute for Advanced Architecture of Catalonia in Barcelona, have been developing self-cooling building material. It uses ‘hydrogel’ which absorbs water. Bubbles of hydrogel is held between two layers of ceramic and when the air around the hydrogel heats up, the water evaporates which cools the air around the gel by around 5 degrees Celsius.

This cool concept (sorry) is similar to the way our own bodies cool down by evaporating water from the skin’s surface in the form of sweat. Using this material in conjunction with a standard AC means you could increase the temperature control up to 5 degree which would cut power usage by 28% and would reduce carbon emissions by about 56.5kg a month on most units.

Here is a video from the inventors:

In the United States, air conditioning makes up around 6% of electricity use. Researchers from the Berkeley Lab have determined that cool walls could lead to yearly heating, ventilation, and air conditioning (HVAC) energy cost savings up to 11% for stand-alone retail stores, 8.3% for single-family homes, as well as 4.6% for medium-sized office buildings.

A passive cooling sysem, white paint has been used in the mediteranean for cenirues. Standard white paint typically reflects about 80% of visible light, but still absorbs ultraviolet (UV) and near-infrared (near-IR) rays, which warm buildings.

Greek builders have been using passive cooling methods for centuries

In a project based on reflective surfaces, researchers at Stanford University reported in Nature that by alternating layers of silicon dioxide and hafnium dioxide, they could create a reflective surface that stayed 5°C cooler than the surrounding air.

In Australia, applied physicists at the University of Technology Sydney reported in 2015 that a cool roofing material made of a pair of polymers kept a commercial roof 3°C and 6°C cooler than ambient air in the midday sun and at night, respectively.

A group from The University of Colorado are commercialising another passive radiative cooling plastic film. The new materials from the research team incorporate materials or structures which reflect nearly all the sun’s rays. They also contain polymers that radiate away additional heat without warming the surrounding air.

Applying coatings to building materials remains an issue. Highly reflective compounds can be integrated into shingles and clay tiles for new construction or renovation, but it's harder to come up with options for retrofitting existing buildings.

That's where the new passive cooling paint comes in.

Researchers at Columbia University had been experimenting with making highly reflective materials by adding air voids to plastics. They then studied ways of curing a common polymer from a liquid to a solid thin film that, under certain conditions, turned from transparent to white as it dried, causing it to strongly reflect light. (The same effect explains why snow is white even though ice cubes are transparent.) The researchers then found they could re-create the effect with other polymers and settled on a highly durable commercial polymer called PVDF-HFP.

The polymer starts as a solution in acetone, to which the researchers add water. When painted on a surface, the acetone evaporates, and the polymer and water separates, creating a network of water droplets. Finally, the water also evaporates, leaving a spongelike arrangement of interconnected voids that reflect up to 99.6% of light, including IR, visible, and UV. Painted surfaces remained 6°C cooler than the surrounding air. At the moment the paint is five times the cost of traditional acrylic paints, but the energy savings would outweigh that, not to mention the benefit to the environment.

Also using historical cooling ideas, another research project based in Egypt andrun by Lund University and the German University in Cairo have been developing a passive heating and cooling system with nineteenth century technololgy.

The Trombe wall works a bit like a greenhouse with a sheet of glass on the outside of a building to trap short wavelength sunlight. A dark coating on the external wall warms up and stores thermal energy that (through conduction) heats the air in the room behind the wall. Trombe walls also include vents at the top and bottom of the wall, which allows you to also to ventilate the room. These walls don’t require any electricity to adjust a room’s temperature, just a bit of human effort to open and close the vents at the right time (which could of course be eventually automated depending on the outside temperature.

The Trombe wall ventilation system in St. Katherine, Egypt (Photo: Marwa Dabaieh)

The resarch team managed to reduce the energy used for heating by 94%, and the energy used for cooling by 73% in a residential building. They were also able to retrofit walls of buildings in Sinai - a semi-arid zone with extreme temperature difference between the day and night.

Here is the head researcher discussing the technology:

A thid technology at prototype stage is called SABER - a self-cooling material membrane that is intended to wrap around buildings an perform like naturally cooling ‘skin’.

Under development by BIOM, SABRE also doesn't need an external power source. It consists of micro-scale valves and lenses that open and close by means of sensors that respond to outside conditions such as heat, light and humidity. In this way, the façade of a building, whether it is applied to a small cabin or an enormous stadium, becomes self-regulating.

The Sabre 'Skin" (source: BIOMS)

This self-regulating membrane does not chill the air like air conditioners do, but rather regulates indoor air, with a passive strategy. BIOM wanted to make something like the human skin that can be=reathe - open and close its pores to regulate temperature, humidity and light. The team are hoping to mak this an ultra-low cost technological alternative.

So self-cooling walls are not science fiction! Along with self-cleaning, anti-fungal and antialgal technology, it is very much in late development stage for materials science and we'll likely see new buildings working with these technologies within the decade.

For details on the environemtnal and energy saving benefits of 'cool wall", take a look at this California Energy Commission Report.


City Metric



Lund University


Material District


Science Mag

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